If you want more details about the investing business itself the most condensed and thorough book I've ever read is "The Millionaire Real Estate Investor", by Gary Keller. 

However, I can't think of innovative ways to prevent the AC condenser from being stolen since it's outside of the house, and these bad guys can still jump over the fence and steal the condenser. 

“Some HVAC contractors have a bad habit of connecting condensation lines to vents inside attic or to drains behind trap.

PVC pipe is pretty cheap and easy to install.A 90% and up furnace will also need a drain line for the *furnace* - they pull enough heat out of the exhaust that the water vapor condenses out and doesn't go up the flue. 

Regarding the bathroom moisture, I did replace their nonfunctional bath fan and the next time I visited, I noticed a drastic difference in both the condensation and the smell.  

Correct me if I'm wrong, but that system essentially eliminates the rim board space that we see in conventional western framing, and thus minimizes thermal/condensation issues.

Here is the condensed version of my lesson.In an effort to not be vanilla, let me start off by saying that in my opinion, the way everyone teaches on how to fix and flip is completely wrong.

It also doesn't include losses (I lost a condenser unit to theft on one of the properties last year and then made sure that I replaced not only that unit, but cages around the others in my little portfolio that didn't have them).

Regardless the other posters are right, if you try to put a window or the wrong size into that opening you will be dealing with venting/comfort issues along with condensate draining issues.

I think it was from condensation from the walls being cold and the weather changes.